Summary:
Commit templates are fully configurable on the server now, so we should be doing
validation there, since an install can add or remove fields and change
validation rules. Remove these outdated client validations, as per comment.
Also update the API call to use the 'errors' field, which allows us to show the
user all the parse errors at once. See:
https://secure.phabricator.com/diffusion/P/browse/origin:master/src/applications/conduit/method/differential/parsecommitmessage/ConduitAPI_differential_parsecommitmessage_Method.php;cfaab709df37739b$75
Test Plan:
Ran "arc diff" on a change with multiple errors:
```$ arc diff --conduit-uri=http://local.aphront.com/
Usage Exception: Commit message is not properly formatted:
Error parsing field 'Reviewers': Commit message references nonexistent users:
jjjderp.
Error parsing field 'CC': Commit message references nonexistent users and
mailing lists: jjjderp.
You should use the standard git commit template to provide a commit message. If
you only want to create a diff (not a revision), use --preview to ignore commit
messages.```
Reviewers: btrahan, jungejason
Reviewed By: btrahan
CC: aran, btrahan
Differential Revision: 1154
